1. Field of the Invention
This invention pertains generally to semiconductor direct bonding techniques, and more particularly to a process for establishing permanent bonds at low temperatures by using nitrogen as a bonding agent.
2. Description of the Background Art
Direct (fusion) bonding between layers of silicon and SiO.sub.2 is a known alternative to using organic or inorganic bonding agents. However, in order to achieve a bond of satisfactory strength, the materials must be annealed at temperatures generally greater than 700.degree. C. As a result, direct bonding techniques have been limited to applications wherein the materials to be bonded can withstand a high temperature anneal.
To expand the versatility of direct bonding, nitride layers have been considered as intermediate bonding agents. The effectiveness of using one such nitride layer, Si.sub.3 N.sub.4, has been debated and even dismissed by those skilled in the art. For example, Lasky's article entitled "Wafer Bonding for Silicon-on-insulator Technologies" and published in Applied Physics Letters in January 1986, reported that Si.sub.3 N.sub.4 was not a bondable surface. A similar conclusion was reached by Yamada, Kawasaki, and Kawashima in their article entitled "Bonding Silicon Wafer to Silicon Nitride with Spin-on Glass as Adhesive" and published in Electronic Letters in 1987. On the other hand, Bower, Ismail, Veteran and Marsh reported in an article entitled "Silicon Nitride Direct Bonding" and published in Electronic Letters in July 1990, that an oxidized surface of nitride could be bonded at high temperatures consistent with silicon fusion bonding. Furthermore, Harendt, Hofflinger, Graf and Penteker in their article entitled "Silicon Direct Bonding for Sensor Applications: Characterization of the Bond Quality" and published in Sensors and Actuators in 1991, reported that LPCVD Si.sub.3 N.sub.4 could be direct bonded at temperatures greater than 800.degree. C.
While conventional direct bonding is effective with a high temperature anneal, some materials are unable to withstand such high temperatures. Therefore, high temperature bonding is limited in its application. In addition, problems occur with thermal mismatching in bonded dissimilar materials. Therefore, a need exists for a process whereby direct bonding can be effected using a low temperature anneal.
None of the reported bonding techniques, however, provide for satisfactory bonding of Si.sub.3 N.sub.4 or other nitride layers such TiN at low temperatures; that is, at temperatures below approximately 500.degree. C. Quite remarkably, however, we have overcome the deficiencies in the prior art and opened the door to low temperature direct bonding by developing a process which utilizes a nitrogen based constituent and an activator to render material surfaces hydrophilic and reactive prior to bonding. Not only can we bond nitride surfaces, but any surface which is reactive with nitrogen. As a result, a low temperature anneal can be used to effectuate a strong bond.
The foregoing publications reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these publications teach or render obvious, singly or when considered in combination, applicant's claimed invention.